The present invention relates in general to extruder screw assemblies for extruding polymers and the like, and more particularly to an extruder screw and positive displacement wave pump structure for extruding plastics and the like.
Employment of a screw or the like in an extruder for working wide ranges of solid plastic material into a substantially homogeneous molten state suitable for formation into any desired shape by extrusion or injection into a die or mold has become well known.
Extrusion, injection molding or blow molding with a single screw extruder includes feeding the solid polymeric or plastic material in pellet, chip, powder, or flake form to the feed end of the extruder through a hopper mounted on an opening of the heated barrel in which a screw is rotatably mounted. The screw has at least one helical thread with a minimum clearance to the barrel, integrally formed on the core to create a helical channel, along which the plastic material is moved downstream from the feed end to the discharge end by forces exerted by the rotation of the screw. The solid plastic material fed into the screw channel is compacted into a solid plug or solid bed and the solid bed melts as it travels down the screw channel. The molten plastic material is collected by the wiping action of the thread into a melt pool. The melt pool gradually increases as the solid bed gradually melts, eventually occupying the entire screw channel.
Molten plastic materials have a very high viscosity and a large amount of heat is generated in the melt pool due to shearing of the melt pool by the rotation of the screw. Thus, the melt pool becomes hotter as it travels down the screw channel and often becomes undesirably hot by the time it reaches the discharge end. Heat transfer from the melt pool to the solid bed is inefficient because of the low thermal conductivity of plastic materials and the limited contact area between the melt pool and the solid bed. Increased heat transfer from the hot, molten plastic material in the melt pool to the cold, solid plastic material in the solid bed is highly desirable in order to reduce the temperature of the molten plastic material discharged from the extruder, increase melting capacity of the extruder and the increase energy efficiency of the extrusion process.
Example of extruder screws which have come into substantial use for working plastic material and feeding it by extrusion or injection into a die or mold are found in Robert A. Barr U.S. Pat. No. 3,698,541, Chan I. Chung, U.S. Pat. No. 4,000,884, and more recently an energy efficient extruder screw disclosed in Chung and Barr U.S. Pat. No. 4,405,239. These can be broadly described as a screw having a first or main screw thread and a second screw thread which divides the screw channel into a pair of side by side sub-channels of equal width. The diameter of the second thread is sufficiently smaller than the diameter of the barrel such that its clearance to the barrel allows the plastic material to flow over the second thread. The depths of the two side-by-side sub-channels vary continuously and oppositely along the length of the passages so that the combined passage cross-sectional area of the two sub-channels is maintained constant. As one sub-channel becomes shall in depth with diminishing cross-section area, the other sub-channel becomes deeper correspondingly with enlarging cross-sectional area, so that the plastic material is forced to move from the diminishing sub-channel into the enlarging sub-channel flowing over the second thread. The second thread gives shearing to the plastic material while flowing over it. Such mechanism of moving the plastic material from one sub-channel into the other sub-channel is repeated a number of times.
It is well known in the plastic and rubber industries that such a single screw type of extruder is not a positive displacement device. This means that the restriction to flow downstream of the extruder which creates high pressure at the extruder outlet reduces the pumping rate of the extruder. To obtain higher rates, it is then necessary to run the extruder at higher revolutions per minute. This has the disadvantage of raising the temperature of the extruder, due to the increase shear eventually beyond what is tolerable to the process.
It has been known in the industry that the pressure on the extruder can be reduced to very low levels by using a positive displacement pump between the extruder and process die such as a gear pump. Driven separately, this is able to pump against very severe flow restrictions, where high pressures occur, and yet provide very low pressure at the pump inlet, which is the extruder outlet, enabling the extruder to perform at much higher revolution rates. However, the gear pump drive system and the in-line space it takes up makes it an expensive and troublesome addition and a real problem to add to existing lines.
In my prior application identified above, I have disclosed an assembly involving a single screw type extruder screw and positive displacement pump assembly, wherein the extruder screw at its discharge end is coupled to and drives a Moineau type pump, designed to keep the extruder head pressure low while the positive displacement Moineau type pump provides the pumping force to overcome the downstream process restrictions to flow. However, certain disadvantages have been identified as an inherent property of that type of construction. With the Moineau type pump, either the rotor or the stator must be free or orbit around the center of rotation of the input shaft. This is because of the nature of the Moineau type pump wherein the confronting services of the rotor and the stator are rounded helical thread configurations of different pitch providing pumping pockets which progress longitudinally from the input to the output end of the positive displacement pump section. Because of the necessity of enabling either the rotor or the stator, usually the rotor, to orbit around the center of rotation of the input shaft, a rather complex coupling must be provided between the output or discharge end of the extruder screw section and the input end of the positive displacement pump section to allow this relative movement of the center of rotation of the positive displacement pump rotor relative to the center of rotation of the extruder screw. The design of this coupling to allow the rotor to orbit the input shaft is complicated in an extruder, because the thrust exerted on the rotor at the output end, due to the high pressure which occurs there, requires the coupling to be capable of absorbing that pressure. Since the coupling in that arrangement is entirely surrounded by a molten polymer at about 350.degree. to 600.degree. F. at about 500 to 1200 psi, the lubrication of the coupling becomes a real problem. Using the polymer which is flowing through the extruder screw assembly to lubricate the coupling is possible, but creates the added disadvantage that if there is any retention of polymer in the coupling, it will degrade in time and can sluff off, causing contamination of the extruded product.
Furthermore, the coupling itself requires a certain length to accommodate the orbiting which is necessary for the rotor, introducing space requirements because of the length necessary for the coupling as well as the positive displacement pump section. In a restricted space such as occurs in an extruded barrel installation where one wishes to minimize the length of the barrel occupied by the pump portion to maximize effective screw length for the extruder screw section to obtain greater output capacity (melting capacity), the elimination of the need for a flexible coupling introduces a real advantage.
Also, the volumetric capacity of the Moineau type positive displacement pump per revolution of the input shaft, in this case the extruder screw section, is fixed by the design of both the rotor and stator of the Moineau type pump. It is desirable to be able to change the volumetric capacity of the positive displacement pump section of an extruder scew and pump assembly, for example by using a rotor with a different pitch so that the ability to readily substitute a rotor with a different pitch and achieve a desired change of volumetric capacity becomes a highly desirable advantage.
An object of the present invention, therefore, is the provision of an extruder screw and positive displacement pump assembly of novel design eliminating the need for a coupling between the extruder screw section and the pump section capable of accommodating orbiting of the pump section rotor relative to the axis of rotation of the extruder screw, and which obviates the disadvantages and provides the advantages described in the foregoing discussion.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description taken in conjunction with accompanying drawings illustrating a preferred embodiment of the inventions.